There exists a long felt need for the transmission of power to a remote location without the need for a physical wire connection. This need has become important in the last few decades, with the popularization of portable electronic devices operated by batteries, which need recharging periodically. Such mobile applications include mobile phones, laptops, cars, toys, wearable devices and hearing aids. Presently, the capacity of state of the art batteries and the typical battery use of a smart phone intensively used may be such that the battery may need charging more than once a day, such that the need for remote wireless battery recharging is important.
Battery technology has a long history, and is still developing. In 1748 Benjamin Franklin described the first battery made of Leyden jars, the first electrical power source, which resembled a cannon battery (hence the name battery). Later in 1800, Volta invented the copper zinc battery, which was significantly more portable. The first rechargeable battery, the lead acid battery, was invented in 1859 by Gaston Plante. Since then the energy density of rechargeable batteries has increased less than 8 times, as observed in FIG. 1, which shows the energy density, both in weight and volume parameters, of various rechargeable battery chemistries, from the original lead acid chemistry to the present day lithium based chemistries and the zinc-air chemistry. At the same time the power consumed by portable electronic/electrical devices has reached a point where several full battery charges may need to be replenished each day.
Almost a century after the invention of the battery, in the period between 1870 and 1910, Tesla attempted the transmission of power over distance using electromagnetic waves. Since then, many attempts have been made to transmit power safely to remote locations, which can be characterized as over a distance significantly larger than the transmitting or receiving device. This ranges from NASA, who conducted the SHARP (Stationary High Altitude Relay Platform) project in the 1980s to Marin Soljacic, who experimented with Tesla-like systems in 2007.
Yet, to date, only three commercially available technologies allow transfer of power to mobile devices safely without wires namely:                Magnetic induction—which is typically limited in range to just a few mm;        Photovoltaic cells—which cannot produce more than 0.1 Watt for the size relevant to mobile phones when illuminated by either solar light or by available levels of artificial lighting in a normally (safe) lit room; and        Energy harvesting techniques—which convert RF waves into usable energy, but cannot operate with more than 0.01 W in any currently practical situation, since RF signal transmission is limited due to health and FCC regulations.        
At the same time, the typical battery of a portable electronic device has a capacity of between 1 and 100 Watt*hour, and typically requires a daily charge, hence a much higher power transfer at a much longer range is needed.
There is therefore an unmet need to safely transfer electrical power, over a large field of view and a range larger than a few meters, to portable electronic devices, which are typically equipped with a rechargeable battery.
A few attempts to transfer power in residential environments, using collimated or essentially collimated, electromagnetic waves, especially laser beams, have been attempted. However, commercial availability of such products to the mass market is limited at the current time. A few problems need to be solved before such a commercial system can be launched:                A system should be developed which is safe.        A system should be developed which is cost effective.        A system should be developed which is capable of enduring the hazards of a common household environment, including contamination such as dust and fingerprints or liquid spills, vibrations, blocking of the beam, unprofessional installation, and periodic dropping onto the floor.        
Currently allowed public exposure to transmitted laser power levels are insufficient for providing useful amount of power without a complex safety system. For example, in the US, the Code of Federal Regulations, title 21, volume 8, (21 CFR §8), revised on April 2014, Chapter I, Subchapter J part 1040 deals with performance standards for light emitting products, including laser products. For wavelengths outside of the visible range, there exist, class I, class III-b and class IV lasers (class II, IIa, and IIIa are for lasers between 400 nm and 710 nm, e.g. visible lasers). Of the lasers outside the visible range, class 1 is considered safe for general public use and classes IIIb and IV are considered unsafe.
Reference is now made to FIG. 2 which is a graph showing the MPE (maximal permissible exposure value) for a 7 mm. pupil diameter, for class I lasers, according to the above referenced 21 CFR §8, for 0.1-60 seconds exposure. It can be seen from the above graph that:                (i) The maximum permissible exposure levels generally (but not always) increase with wavelength, and        (ii) Even if the laser is turned off some 0.1 second after a person enters the beam, in order to meet the requirement specified in 21 CFR §8, no more than 1.25 W of light can be transmitted, and that at wavelengths longer than 2.5μ, with the limit orders of magnitude less at shorter wavelengths.        
Thus, without some kind of safety system, only a few milliwatts of laser power are allowed to be transmitted, which even if completely converted back to electricity, would supply significantly less power than the power needed to charge most portable electronic devices. A cellular phone, for example, requires from 1 to 12 W for charging, depending on the model.
To transmit power higher than that of class 1 laser MPE, a safety system is needed. None, to the best of the applicants' knowledge, has yet been commercialized for transmitting significant power levels in residential environment accessible to untrained people.
Building of a transmission system having a robust safety system is difficult. The required detection levels are very small compared to the power that needs to be transmitted, the environment in which the system operates is uncontrolled and many unpredictable scenarios may happen while working.
It is well known in the art that fingerprints and dust scatter laser light and that transparent surfaces reflect or scatter it. If high power is to be transferred, then a class IV (or IIIb) laser would be needed, which would require a reliable safety system. For Class IV lasers, even scattered radiation from the main beam is dangerous. According to the 21 CFR §8, as revised on April 2014, Chapter I, Subchapter J part 1040, lasers emitting between 400 nm and 1400 nm, having more than 0.5 W beam output, are usually considered class IV lasers for exposures above 0.5 sec, and even scattered radiation from such lasers may be dangerous. Such lasers are required to have a lock key and a warning label similar to that shown in FIG. 3, where it is noted that the warning relates to “scattered radiation” also, and the user of the laser is usually required to wear safety googles and is typically a trained professional, all of these aspects being very different from the acceptable conditions of use of a domestically available laser power transmission system for charging mobile electronic devices.
The prior art typically uses anti-reflective coatings on surfaces to prevent such reflections, in combination with elaborate beam blocking structures to block such reflections, should they nevertheless occur. However, the AR-coating solution used in the prior art is prone to failure from dust or spilled liquid deposited on its surface, or from coating wear and tear, such as from improper cleaning. Additionally, the beam block solution typically limits the field of view of the system severely, and is bulky compared to the dimensions of modern portable electronic devices.
The prior art therefore lacks a reliable and “small footprint” mechanism to prevent scattering and reflections from the power beam in unwanted directions. Such scattering and reflections may be caused either by a transparent surface inadvertently placed between the transmitter and the receiver, and the optical characteristics of that transparent surface may arise from a vast number of different transparent materials, or from liquid spills and fingerprints which may be deposited on the external surfaces of the system, typically on the front surface of the receiver.
A third problem with the solutions suggested in the prior art, is that such safety systems generally require a mechanism to guarantee good alignment of the power beam system and the safety system such that both systems are boresighted on the same axis until the power beam diverges enough or is attenuated enough (or a combination of these factors and any other factors) so that it no longer exceeds safety limits. This is extremely difficult to achieve with a collimated class IV or IIIb laser beam, which typically expands very little with distance and thus exceeds the safety limit for a very long distance.
One prior art principle of operation used to build such a safety system is the optical detection of transparent surfaces that may be positioned in the beam's path. However transparent surfaces that may enter the beam path may be made from a vast number of different transparent materials, may be antireflection AR coated or may be placed in an angle close to Brewster's angle so they are almost invisible to an optical system unless they absorb the beam. However, since light absorption levels for each material are different, and may even be negligible, and since building an optical system that relies on optical absorption will be highly material specific, and since the number of available materials is extremely large, such a system is likely to be complex, large and expensive, and unless properly designed, is likely to be unreliable, especially when considering that it is meant to be a critical safety system. Relying on the reflections to provide detectable attenuation of the beam is also problematic, as the surfaces may be coated by an anti-reflective coating or positioned in a near Brewster angle to the beam, such that the reflection may be minimal for that particular position of the surface.
Another limitation of prior art systems is that they are typically use lasers having good beam quality (low m2 value) combined with large optics in order to yield high efficiency (for example U.S. Pat. No. 6,407,535 B1 and U.S. Pat. No. 6,534,705 B2 uses a wide aperture for the laser beams) while U.S. Pat. No. 5,260,639 uses a wavelength of 0.8 um to allow for small optics, reducing cost and size of the optical system.
There therefore exists a need for a laser power transmission system with built-in safety features, which overcomes at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.